Sustained plankton blooms under open chaotic flows

We consider a predator-prey model of planktonic population dynamics, of excitable character, living in an open and chaotic fluid flow, i.e., a state of fluid motion in a region from which fluid trajectories are continuously escaping, but such that it contains a chaotic subset. During the time trajectories spend in this chaotic area they are characterized by a positive maximum Lyapunov exponent, so that close fluid particles separate exponentially. Despite that excitability in the predator-prey dynamics is a transient phenomenon and that fluid trajectories are continuously leaving the system, there is a regime of parameters where the excitation remains permanently in it, given rise to a persistent plankton bloom. This regime is reached when the time scales associated to fluid stirring become slower than the ones associated to biological growth. (c) 2004 Elsevier B.V. All rights reserved.